The UniCharCount type is the number of characters in backing store memory that correspond to the length of the range of text within a text layout object to be operated on. Functions that perform layout operations on text do so within this range of text, not the entire text buffer. If you want the range of text to span the end of the text buffer, you should pass the constant kATSUToTextEnd, described in Text Length Constant.
typedef UInt32 UniCharCount;
